Portable devices are very popular in our society. Common use of such devices demands the availability of portable source with which to power them. The variety of devices also demands that the power be accurately controlled. Even devices that are not portable benefit from the generation of power in a highly controlled manner. The power generators are operated while connected to an electrical storage unit such as a battery or a capacitor. Upon actuation, generated electrical pulses are delivered to a storage device. The storage device is then used as power source for another device.
Research for non-conventional sources of energy is gaining paramount importance owing to the disappearing natural resources and an increased concern for environmental safety. Over the last decade, there has been a renewed interest in “direct conversion” of heat into electricity with the discovery of new materials and structures with enhanced thermionic or thermoelectric properties. Thermionic conversion depends on the production of a current due to the flow of electrons emitted from a hot cathode. In thermoelectric conversion, a potential is developed across the material when the two ends of the material are kept at two different temperatures. Such conversions are attractive as they involve no moving parts and release no harmful byproducts to the environment. A semiconductor based “Thermal Diode”, which consists of a thin thermionic emitter layer on the hot side of a thick near intrinsic semiconductor thermoelectric. Such a device has been reported to show a significant increase in the conversion efficiency as it combines the conversion due to thermionic emission as well as thermoelectric effects. Further enhancement in the conversion efficiency has been reported by blocking the reverse ohmic current by placing a hindering layer at the collector side of the thermal diode. One problem with this design is the eventual disposal of the device. While the conversion itself does not produce any byproducts, the materials used for the device, such as HgCdTe and InSb, are toxic in nature. Also, conventional microfabrication techniques cannot be applied to fabricate such a device.
Explosives are known to produce power, but their use is associated with uncontrolled generation of thermal and mechanical forces. Such forces are difficult to harness to power electrical devices. In the past few years, several studies were performed on investigating the power generation ability of piezoelectric materials. In order to produce electrical power from vibrations, thick-film piezoelectric technologies were used and maximum power output of about 2 mW was obtained under a resonant frequency. It is calculated that the power generated from a 1 cm2 piezoelectric plate can supply microwatt to milliwatt of power for in vivo bio Microelectro Mechanical Systems (MEMS) applications. The power generated from two types of circular diaphragm structures by varying the thickness ratio was also compared. Recently, three dimensional analyses of a parallel piezoelectric bimorph and triple layer piezoelectric actuators were also done. Normally, as the power generated from piezoelectric materials is too small to be used in practical applications, it is necessary to store the energy by using energy storage devices. Thus, a bridge rectifier and a capacitor to store the energy generated from a piezoelectric generator was used, whilst the efficiency of the generator was evaluated. A piece of lead zirconium titanate (PZT) unimorph and polyvinylidene fluoride (PVDF) stave mounted in a pair of sneakers to generate power during walking, then the energy collected was used to power a RF tag system. The power from a piezoelectric material using a DC-DC converter with an adaptive control algorithm was maximized. With an open circuit voltage of about 95 V, the power stored in the battery can be four times higher than direct charging. Also, experimental investigation for the possibility of harvesting power from a PZT beam (where the energy produced was stored in a 40 mAh nickel metal hydride battery) was performed. Furthermore, investigation for the power harvesting of PZT fibers via strain energy was done and their applications in wireless sensor networks were discussed.
Pulse power generators have been built using explosives to generate shock-waves and depolarize PZT crystals. This device uses a detonator filled with C4 to accelerate an aluminum flyer plate into a PZT disc. The impact of the flyer plate causes the PZT crystal to depolarize, which generates a large voltage pulse. They find a linear relationship between the thickness of the PZT and the generated voltage.